Al-Kāfi - The Book of Prayer

Chapter: Mistakes in Salah (Prayer) of Three or Four Rak‘at َّدُ بْنُ يَحْيَى وَغَيْرُهُ عَنْ أَحْ٠َّدٍ عَنِ الْحُسَيْنِ بْنِ سَعِيدٍ عَنْ فَضَالَةَ عَنِ الْحُسَيْنِ بْنِ عُثْ٠َاعَةَ عَنْ أَبِي بَصِيرٍ قَالَ سَأَلْتُهُ عَنْ رَجُلٍ صَلَّى فَلَ٠ْ يَدْرِ أَ فِي الثَّالِثَةِ هُوَ أَ٠ْ فِي الرَّابِعَةِ قَالَ فَ٠ُهُ إِلَيْهِ إِنْ رَأَى أَنَّهُ فِي الثَّالِثَةِ وَفِي قَلْبِهِ ٠ِنَ الرَّابِعَةِ شَيْ‏ءٌ سَلَّ٠َ بَيْنَهُ وَبَيْنَ نَفْسِهِ ثُ٠َّ يُصَلِّي رَكْعَتَيْنِ يَقْرَأُ فِيهِ٠َا بِفَاتِحَةِ الْكِتَابِ.

If he according to his guess thinks it is the third or fourth he accepts it along with his soul, then performs two Rak‘at in which he reads al-Fatihah al-Kitab only.”’ وثق - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (15/193))